Galvanized steel sheets are widely used in a wide range of fields, mainly, automobile body applications. In such applications, galvanized steel sheets are press-formed and painted before use.
However, galvanized steel sheets have a drawback in that they have poor press formability compared to cold-rolled steel sheets. This is because sliding resistance of galvanized steel sheets in press dies is larger than that of cold-rolled steel sheets. That is, a galvanized steel sheet does not smoothly flow into a press die at a portion where the sliding resistance between the die and the bead is large, and the steel sheet is likely to break.
A method of applying highly viscous lubricant oil has been widely employed as a method of improving press formability of galvanized steel sheets used. This method has a problem in that oil shortage during press forming results in instable press performance, for example. Accordingly, it is highly desirable to improve press formability of the galvanized steel sheets themselves.
In recent years, attempts have been made to simplify the production processes and reduce the amounts of substances of concern generated in the production processes. In particular, progress has been made on reducing the line length of the alkali degreasing process, which is a process that precedes a painting process, and decreasing the temperature of the working environment in the alkali degreasing process. Steel sheets that have excellent degreasability and do not adversely affect the painting process despite such severe conditions are in demand.
Accordingly, steel sheets that have excellent press formability as well as excellent degreasability under alkali degreasing conditions more stringent than in the related art are desirable as the galvanized steel sheets for automobiles.
One example of a method for improving press formability is a technique of forming a lubricant film or an oxide layer on a surface of a galvanized steel sheet.
Patent Literature 1 discloses a technique of improving press formability and chemical conversion ability by causing Ni oxides to occur on a surface of a zinc coated steel sheet by an electrolytic treatment, an immersion treatment, an application-oxidation treatment, or a heat treatment.
Patent Literatures 2 and 3 each disclose a technique of suppressing adhesion between a hot-dip galvannealed coating layer and a press die and improving slidability by causing a hot-dip galvannealed steel sheet to come into contact with a sulfuric acid acidic solution so that an oxide layer mainly composed of Zn oxide is formed on the steel sheet surface.
An example of the method for improving degreasability is to wash a steel sheet with an alkaline solution or a solution containing phosphorus (P).
Patent Literature 4 discloses a technique of improving degreasability by washing a surface of a hot-dip galvannealed steel sheet with an alkaline solution.
Patent Literature 5 discloses a technique of improving degreasability by washing a surface of a hot-dip galvannealed steel sheet with a solution containing P.